Quasiperiodic ∼5–60 s fluctuations of VLF signals propagating in the Earth-ionosphere waveguide: a result of pulsating auroral particle precipitation?

Subionospheric very low frequency and low-frequency (VLF/LF) transmitter signals received at middle-latitude ground stations at nighttime were found to exhibit pulsating behavior with periods that were typically in the ∼5–60 s range but sometimes reached ∼100 s. The amplitude versus time shape of the pulsations was often triangular or zigzag-like, hence the term “zigzag effect.” Variations in the envelope shape were usually in the direction of faster development than recovery. Episodes of zigzag activity at Siple, Antarctica (L ∼4.3), and Saskatoon, Canada (L ∼4.2), were found to occur widely during the predawn hours and were not observed during geomagnetically quiet periods. The fluctuations appeared to be caused by ionospheric perturbations at the ∼ 85 km nighttime VLF reflection height in regions poleward of the plasmapause. We infer that in the case of the Saskatoon and Siple data, the perturbations were centered within ∼500 km of the stations and within ∼ 100–200 km of the affected signal paths. Their horizontal extent is inferred to have been in the range ∼50–200 km. The assembled evidence, supported by Corcuffs [1996] recent research at Kerguelen (L ∼3.7), suggests that the underlying cause of the effect was pulsating auroral precipitation. The means by which that precipitation produces ionospheric perturbations at 85 km is not yet clear. Candidate mechanisms include (1) acoustic waves that propagate downward from precipitation regions above the ∼ 85 km VLF reflection level; (2) quasi-static perturbation electric fields that give rise to E×B drifts of the bottomside ionosphere; (3) secondary ionization production and subsequent decay at or below 85 km. Those zigzag fluctuations exhibiting notably faster development than recovery probably originated in secondary ionization produced near 85 km by the more energetic (E >40 keV) electrons in the incident electron spectrum

Quasi-periodic particle precipitation and Trimpi activity at Halley, Antarctica

The relationship between quasi-periodic VLF emissions and micropulsations is briefly reviewed, and then discussed with reference to an event recorded at Halley, Antarctica, on day 257 in 1986. VLF emissions at 2 kHz with a quasi-period of 9 s were observed simultaneously with Pi1 and Pe1 micropulsations. Also observed was a quasi-periodic Trimpi event on the amplitudes and phases of the VLF transmitters NAA and NSS. It is deduced that the VLF emissions are modulated in the generation region by a hydromagnetic wave, giving rise to particle precipitation. The emissions are also modulated by the bounce period of the generating particles. The Trimpi effect is due to 120 keV electrons being precipitated into the lower ionosphere by the interaction with the VLF emissions. This event shows that the Trimpi effect can be used to detect particle precipitation occurring during the ULF/VLF interaction, and can give information which helps to define the mechanism reponsible for the interactio

Geomagnetically induced currents in the UK: geomagnetic variations and surface electric fields

The geomagnetically induced current (GIC) risk to the power transmission grid in the United Kingdom is discussed with reference to an example of a geomagnetic storm during which GICs were suspected of causing abnormal transformer behaviour. A simple measure of the power of the magnetic field variation, the hourly standard deviation (HSD) in the north or east horizontal component, is used to determine the general risk to the UK power grid from rapid magnetic variations, according to season and local time. Monitoring and forecasting of HSD may be a useful means of gauging the likely risk to high-cost power engineering equipment. A simpli,ed but representative three-dimensional geological model of the UK landmass and surrounding seas is used to provide an indication of the surface electric field for various amplitudes and orientations of external magnetic field variations. It is found that the resistivity contrast between seawater and the onshore geology, particularly around the Scottish metamorphic terranes, produces enhanced electric ,elds at coastal sites. These are as much as 4 V/km for a 1 A/m (or 1257 nT) external field with 10 min period

Increased magnetic storm activity from 1868 to 1995

The aa index provides the longest continuous data set which can be used in the analysis of magnetospheric and ionospheric phenomenology. All phases of the solar cycle show increases in activity since cycle 14. The activity increase does not appear to be associated with any instrumental, ionospheric or magnetospheric effects. Barely significant effects (in terms of the results presented in this paper) have been identified in the long-term change in magnetic latitude of the observatory sites, the positions of high-latitude ionospheric features such as the cusp, and ionospheric Pedersen and Hall conductivities due to changing magnetic field orientation and strength. The prime cause of the change in geomagnetic activity is an increase in solar activity. The number of storms at solar minimum has typically increased by 40% more than the other phases. This is principally due to increased recurrent storm activity to such an extent that conditions at minimum in recent cycles could be thought of as being more representative of the declining phase

Observation of a decrease in mid-latitude whistler mode signal occurrence prior to geomagnetic storms

VLF whistler mode signals received in 1986–1992 at Faraday, Antarctica (65° S, 64° W) and Dunedin, New Zealand (46° S, 171° E) show night-long decreases in occurrence which may be caused by changes in F-region absorption levels. The whistler mode occurrence normally decreases for one night and can only be detected during periods when whistler mode activity lasting several hours per night is usual. Decreases in occurrence are observed more often at Dunedin than at Faraday, probably due to long sub-ionospheric paths that result in weaker signals being received at Dunedin. The decreases in occurrence appear to be associated with solar disturbances and often occur a day before the onset of geomagnetic activity. Several of the events recur with a 27-day cycle that coincides with favourably placed solar coronal holes